Outflow from the nucleus accumbens to the pedunculopontine tegmental nucleus: a dissociation between locomotor activity and the acquisition of responding for conditioned reinforcement stimulated by d-amphetamine.

Output of neuronal information from the nucleus accumbens to the ventral pallidum is known to be a critical pathway in the expression of locomotion and incentive-related behaviour. Some signals from this structure are relayed forward through the dorsomedial nucleus of the thalamus to the medial prefrontal cortex, but the other major pathway from this site is a descending innervation to the pedunculopontine tegmental nucleus. Information carried by these descending neurons has been linked with both the output of locomotor activity and incentive-related information. Previous studies carried out in this laboratory have shown no changes in locomotor activity--either spontaneous or in response to systemic administration of d-amphetamine or apomorphine--in rats with excitotoxic lesions of the pedunculopontine tegmental nucleus. The present experiments compare the effects of ibotenate lesions of this nucleus in tests of locomotor activity or the acquisition of responding with conditioned reinforcement, following injections of d-amphetamine directly into the nucleus accumbens. In general agreement with previous results, ibotenate lesions of the pedunculopontine tegmental nucleus did not alter locomotion stimulated directly from the nucleus accumbens. However, comparable lesions in a group of trained rats produced an array of deficits in the conditioned reinforcement paradigm. Most notably, these rats directed their attention almost entirely towards pressing the levers (practically ignoring the food-hopper panel), but did not appear to be able to discriminate between them, while controls focused almost all their efforts on pressing the reinforcing lever (virtually ignoring the non-reinforcing lever) and the food-hopper panel. These results indicate that pedunculopontine tegmental nucleus lesions disrupt an element of reward-related responding, but do not affect the production of locomotor activity. This highlights the unlikely existence of specific "locomotion-inducing" centres in the mesencephalon and implicates the pedunculopontine tegmental nucleus in the formation of stimulus-reward associations. These data are discussed with respect to a role for the pedunculopontine tegmental nucleus in response selection.